(1) Field of the Invention
The invention is concerned with electrodeposition of polymers in aqueous dispersion onto conductive surfaces wherein the article which comprises such surfaces is the cathode in an electric current carrying loop.
(2) Description of the Prior Art
The usual procedure for coating of polymers, which are dispersed in aqueous media, onto articles has been carried out by making the article to be coated the anode. Most such anodic depositions employ the carboxyl as the functional group which assists in the transport of the polymer to the electrode. It is generally agreed that during the deposition of the polymer on the electrode surface, hydrogen ions react with the carboxyl anion to form a water-insoluble acid. Hence, the carboxyl groups remain in the polymer thereby providing relatively hydrophilic points of attack for water. Additionally, oxidation occurs at the anode. When the anode, i.e., the article to be coated is a metal, metal ions are produced. This production of metal ions not only is destructive to the anode but may cause discoloration and also tends to coagulate the polymer in an aqueous dispersion, thus forming useless sludge. Furthermore, the charge on the polymer is pH dependent and, hence, the pH must be controlled carefully to get satisfactory coating. However, at the cathode, reduction occurs rather than oxidation. Hence, when the article to be coated is made the cathode, formation of metal ions cannot occur at that electrode. There previously has been some recognition that a cathodic electrodeposition system would be desirable. However, there are also problems with the known cationic systems. In general, especially with amine and ammonium type surfactants, the known cationic methods are carried out at low pH. Corrosion of metals becomes a problem at low pH. Such a method, for example, is shown in U.S. Pat. No. 3,159,558 in which the process is carried out at a pH of from about 2 to about 6. The cations, which provide the means of transport in most of the known cationic electrophoretic systems, remain with the polymer after deposition and continue to be points of moisture sensitivity. Furthermore, in the weak base systems, the pH and the conductivity are interdependent and cannot be easily varied independently.